1. Field of the Invention
The present invention relates to an organic electroluminescence device.
2. Description of Related Art
When voltage is applied on an organic electroluminescence device (hereinafter, referred to as an organic EL device), holes and electrons are respectively injected into an emitting layer from an anode and a cathode. The injected electrons and holes are recombined in an emitting layer to form excitons. Here, according to the electron spin statistics theory, singlet excitons and triplet excitons are generated at a ratio of 25%:75%. In the classification according to the emission principle, in a fluorescent EL device which uses emission caused by singlet excitons, the limited value of an internal quantum efficiency of the organic EL device is believed to be 25%. On the other hand, in a phosphorescent EL device which uses emission caused by triplet excitons, it has been known that the internal quantum efficiency can be improved up to 100% when intersystem crossing efficiently occurs from the singlet excitons.
A technology for extending a lifetime of a fluorescent organic EL device has recently been improved and applied to a full-color display of a mobile phone, TV and the like. However, an efficiency of a fluorescent EL device is required to be improved.
Based on such a background, a highly efficient fluorescent organic EL device using delayed fluorescence has been proposed and developed. For instance, Document 1 (International Publication No. WO2010/134350) discloses an organic EL device using TTF (Triplet-Triplet Fusion) mechanism that is one of mechanisms for delayed fluorescence. The TTF mechanism utilizes a phenomenon in which singlet excitons are generated by collision between two triplet excitons.
By using delayed fluorescence by the TTF mechanism, it is considered that an internal quantum efficiency can be theoretically raised up to 40% even in fluorescent emission. However, as compared with phosphorescent emission, the fluorescent emission is still problematic on improving efficiency. Accordingly, in order to enhance the internal quantum efficiency, an organic EL device using another delayed fluorescence mechanism has been studied.
For instance, TADF (Thermally Activated Delayed Fluorescence) mechanism is used. The TADF mechanism utilizes a phenomenon in which inverse intersystem crossing from triplet excitons to singlet excitons is generated by using a material having a small energy gap (ΔST) between the singlet level and the triplet level. An organic EL device using the TADF mechanism is disclosed in Document 2: “Expression of Highly-Efficient Thermally-Activated Delayed-Fluorescence and Application thereof to OLED” Organic EL Symposium, proceeding for the tenth meeting edited by Chihaya Adachi et al., pp. 11-12, Jun. 17-18, 2010. In the organic EL device of Document 2, a material having a small ΔST is used as a dopant material to cause inverse intersystem crossing from the triplet level to the singlet level by heat energy. It is considered that the internal quantum efficiency can be theoretically raised up to 100% even in fluorescent emission by using delayed fluorescence by the TADF mechanism.
Although the organic EL device disclosed in Document 2 exhibits the maximum luminous efficiency at 0.01 mA/cm2 of a low current density area, so-called roll-off is generated to decrease a luminous efficiency in a practically high current density area from approximately 1 mA/cm2 to 10 mA/cm2.
Accordingly, it is considered that many practical problems in using delayed fluorescence by the TADF mechanism are left unsolved, among which improvement in the luminous efficiency in the practically high current density area has been particularly demanded.